The invention relates to a method and apparatus for producing ingots of unalloyed and alloyed steels having an improved primary crystallization, reduced ingot segregation and a reduced content of non-metallic inclusions, wherein, in a known way (German Auslegeschrift No. 1,812,102), molten steel is poured into a mould first, whereupon a slag mixture is supplied onto it and this slag bath in turn is supplied with energy via one or more consumable electrodes during the solidification of the steel. The energy advantageously corresponds to at least 120 kilowatt-hours per metric-ton of ingot weight. Such a mould preferably is a mould merely subjected to the surrounding atmosphere, i.e. a mould like that in the German reference which is not cooled by special means, such as a liquid.
For carrying out this method an apparatus can be used in which a top part accommodating the slag bath and having cooled walls or walls lined with refractory material is placed on the upper rim of the mould or on the ingot.
An important prerequisite for producing high-quality ingots is that on the bordering face between the molten metal and slag, temperatures prevail in the slag bath up to the area near the wall of the top part above the solidifying ingot, which temperatures are above the liquidus temperature of the cast steel, and that, during the solidification of the ingot, no changes in the flow processes within the molten ingot part disadvantageously influencing the crystallization of the ingot will occur when applying this method.
When producing ingots with large diameters, difficulties may arise when the slag bath is to be heated and kept at high temperatures with a single electrode. There is an intensive heat decrease from the electrode towards the top part device, and therefore it is necessary to strongly heat the slag in the region of the immersed electrode in order to be able to supply the necessary energy and to make use of the great slag bath heights for obtaining a sufficient distribution of heat. The intensive heat decrease from the electrode towards the top part device in connection with the requirement that, at least at the beginning of the process, the slag layer on the total bordering face between the molten ingot and the slag bath is to be kept at temperatures above the liquidus temperature, necessitates using a consumable electrode with a large diameter and great slag bath heights. However, when supplying the electric energy necessary for maintaining the most favourable slag bath temperature, the disadvantage will arise that electrodes with large diameters have a considerably higher melting-off rate than would correspond to the shrinking of the ingot when solidifying. In other words, more metal is dripping from the electrode than the process would require, thus leading to disadvantages as regards the economy of the process as well as the quality of the ingot produced. Also with small changes in the electric energy supply, detrimental flows in the molten ingot part will be created by great slag bath heights, thus causing inhomogeneities within the ingot structure.
Sufficient temperature distribution when using one or more electrodes has so far been achieved only by great slag bath heights and high electric energy supply, which again reduces the economy of the process, increases the machinery required, e.g. the slag melting aggregates, and requires special measures as regards the energy supply to the slag bath during the solidification of the ingot, in order to produce high-quality ingots.